Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to an optical disk resonator that includes a composite silicon structure.
Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, large density, low latency and low power consumption for inter-chip and intra-chip connections. In the last few years, significant progress has been made in developing low-cost components for use in inter-chip and intra-chip silicon-photonic connections, including: high-bandwidth efficient silicon modulators, low-loss optical waveguides, wavelength-division-multiplexing (WDM) components, and high-speed CMOS optical-waveguide photo-detectors.
Electro-optical modulation in silicon can be implemented by an electro-absorption or an electro-optical effect. In order to achieve high modulation efficiency, electro-absorption-based optical modulators generally involve the hybrid integration of other materials (such as germanium or germanium-silicon alloy) into a silicon-on-insulator (SOI) platform. But these intensity optical modulators typically have very high optical waveguide loss because of indirect band-gap absorption, which can limit the optical modulator length and, thus, the overall performance. Alternatively, optical modulation in silicon can also be implemented by exploiting the free-carrier dispersion effect of silicon, in which the concentration of free charges in silicon changes the real and imaginary parts of the index of refraction. This type of phase optical modulator is often implemented in a Mach-Zehnder interferometer (MZI) or a resonator structure that converts a phase change into an intensity modulation.
In an electro-optical silicon optical modulator, electrical manipulation of the charge density interacting with light can be achieved through one of three major mechanisms: carrier injection, carrier depletion and carrier accumulation. Carrier injection-based silicon optical modulators typically use forward-biased pin diodes to inject free carriers into the intrinsic region with low drive voltage. This modulation technique is usually very efficient and has low optical loss. However, the intrinsic speed of carrier injection-based silicon optical modulators is often very low because of minority-carrier diffusion. Consequently, signal pre-emphasis is usually required to achieve a speed of 10 Gb/s or more.
Carrier depletion-based silicon optical modulators, in which the depletion region of a pn junction is modulated under reverse bias, are typically favored for high-speed operation and low-power modulation. However, this type of optical modulator usually has a low modulation efficiency. Larger modulation often requires higher p/n doping, which can increase the insertion loss.
Carrier accumulation-based optical modulators typically use a metal-oxide-semiconductor (MOS)-capacitor type of structure (such as using polysilicon, a dielectric layer and the silicon layer in an SOI platform) to accumulate free carriers on either side of the dielectric layer (such as an oxide layer). With a thin oxide layer (typically less than 10 nm) inserted in the middle of the optical waveguide, this structure can achieve a large phase shift with small bias voltage. In addition, the MOS capacitor may be operated in an accumulation mode so that the optical modulator bandwidth is not limited by carrier recombination inside the silicon.
However, an MOS-capacitor structure often has its own limitations when used for high-speed optical modulators. In MOS-capacitor type optical modulators based on MZIs, the typical length of the phase-shifting element is in the millimeter range. Although the dynamic operation of carrier-accumulation modulators is not limited by the relatively long minority carrier lifetime, it is often limited by the optical modulator resistance and capacitance. In particular, MOS MZI optical modulators tend to have much higher capacitance, which is associated with the long arms.
Furthermore, a slab optical waveguide structure usually has to be implemented in the MZI to enable the electrical connections to driving pads. In order to achieve high-modulation speed, shallow-etched optical waveguides are typically preferred. However, a thick silicon slab may reduce the optical mode overlap with the electrical charges, and thus may lower the effective change in the index of refraction. Consequently, the slab thickness is typically selected in an attempt to balance the bandwidth and efficiency, which often results in an RC-limited bandwidth.
Additionally, the top silicon layer of MOS-capacitor optical modulators is usually polysilicon, which has much higher loss than crystalline silicon because of defects in the material lattice. For a typical MZI with an arm length of 1 mm, this extra loss from polysilicon scattering can be significant. Consequently, the total insertion loss of these optical modulators is often too high for most applications.
Hence, what is needed is an electro-optical modulator without the above-described problems.